It all went down at the pre-fight media event in L.A. -- when Mosley's GF bent over in front of Mayorga ... and he delivered the low blow ... while cameras were rolling.

1. Shane tried to choke him out. 2. Shane's GF orders him to "kill" Mayorga. 3. Shane's GF offers Mayorga $10,000 to let her punch him in the face.4. Mayorga offers her $25k to let him slap her ass again.

Cheap publicity stunt? Of course ... but it worked. The clip is great.